Lubricating oils have been conventionally used as the lubricating agent for machine tools having a sliding surface portion to be lubricated, and almost no grease has been used for those machine tools. That is because the grease, which is in a semi-solid state shows inferior flowing performance toward the sliding surface than the lubricating oil (in a liquid form), thereby frequently leading to the problem of poor lubrication. In addition, the semi-solid state grease has another disadvantage that it is difficult or impossible for an automatic lubricator to pump the grease.
The lubricating oil can be pumped, but an excess amount of lubricating oil is released while in use. Therefore, large quantities of lubricating oil are used, which produces the problem of increase in consumption cost of the lubricating oil. Further, the lubricating oil causes the problems that scattering of lubricating oil will impair the environment, atmosphere and machine itself, and a mist of lubricating oil will make the working environment worse.
In light of the above, there is an increasing demand for a grease that can be pumped with no waste to reduce the consumption, and minimum scattering.
The sliding surfaces of the machine tool can cause a spontaneous jerking motion, which is called a stick-slip phenomenon when the operation is started. The stick-slip has an effect on the processing precision. When the machine tool is started, the static friction becomes large because of the absence of a lubricating oil film, thereby inducing the stick-slip phenomenon. Theoretically, the thickness of the oil film reaches zero while the sliding surfaces are not operated. Therefore, the above-mentioned stick-slip phenomenon can occur when the operation is started.
The lubricating oil does not readily cause the stick-slip. This is because the lubricating oil has such excellent flowability to the sliding surfaces that the oil film can be formed immediately after the operation is started.
On the other hand, the grease cannot promptly form an oil film because of the poor flowability to the sliding surfaces. Improvement in the stick-slip resistance has been a serious topic in the grease.